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Rising seas may help some coral reefs

Modelling shows reefs with water circulated by tides may have their temperature extremes evened out under climate change. Angus Bezzina reports.

Tidal reefs, such as the Montgomery Reef in Western Australia's Kimberley region, may benefit from rising sea levels.

laurenepbath / Getty Images

There’s no doubt climate change is already damaging coral reefs. Just look at the Great Barrier Reef, where large swathes have been bleached bone-white, thanks to warmer seas. Rising sea levels – estimated to increase by as much as 80 centimetres by 2100 – threaten to worsen these conditions.

But there may be a silver lining.

An Australian team led by oceanographer Ryan Lowe from the University of Western Australia suggests rising sea levels due to climate change could actually help some reef systems by smoothing out temperature extremes.

To untangle how regional ocean warming patterns are driven by the long-term effects of climate change and by the short-term climate patterns such as the El Niño-Southern Oscillation cycle, oceanographers must deal with large-scale data.

But this broad view tends to overlook local effects in specific ecosystems – such as reefs, which vary in terms of the species that live among them and whether their waters are circulated by tides or waves.

So Lowe and his colleagues looked at the tide-dominated reef habitats around north-western Australia and the Kimberley, an area renowned for having some of the largest tides in the world.

They narrowed in on Tallon Island, which sits right at the mouth of King Sound and has one of the three largest tidal ranges in the world.

“Tallon is typical of a lot of the reefs where the reef platforms rise out of fairly deep water and the tidal range at the entrance is about 10 metres, so in some parts of the day you can have tides that are four metres above the reef and then other times it can be four metres below the reef if you look offshore,” Lowe says.

“You essentially get water flowing off the reefs as waterfalls.”

They measured the temperature of the ocean at different depths at different times of the day as well as the air above.

They found heat emitted from the sea surface into the atmosphere is the primary force behind the extreme temperature fluctuations in reef habitats that occupy shallow, low tide waters.

Lowe explains that as such, a rapidly increasing sea level over the course of the rest of the century could decrease local extreme temperature variations in reefs such as the one at Tallon Island.

This is because the higher sea level would increase the amount of cool water pooling on top of the reef at low tide as well as draw out and extend the tide itself, evening out the temperature below.

Still, Lowe says, oceanographers are yet to uncover the local impacts of temperature and temperature extremes on reefs around the world but he is adamant that local analyses of reefs will be much more important than the larger scale rise in temperature of the oceans.

“The temperature extremes that you can see locally in reefs will far outweigh large-scale warming in the oceans. There is a need to scale the model down to actual reefs and the condition that they’ll experience”.